The obtaining of dry air through the use of desiccant dehumidifiers is well-known. These dehumidifiers utilize, as the desiccant, a porous adsorbent such as silica gel, activated alumina and molecular sieves, all of which have an enormous natural affinity for water. As humid air to be processed is passed through a permeable bed of the desiccant material in the dehumidifier, the moisture in the air is condensed out in the pores and capillaries of the desiccant material, and the latent heat of vaporization of the moisture condensed is converted to sensible heat, raising the temperature of the effluent air stream. Extremely low dew points can be obtained in this way, and it is apparently the simplest way of obtaining large volumes of dry air. At some point in time during its use as a drying agent in the dehumidifier, however, the desiccant material reaches a level of saturation with adsorbed moisture at which it requires renewal or regeneration of its adsorptive potency through removal of the adsorbed moisture, whereby it can be reused effectively as a drying agent. Thus, the known desiccant dehumidifiers are generally designed to carry out the required regeneration, either intermittently or continuously and concurrently with dehumidification.
One conventional dehumidifier/regenerator, designed for intermittent duty, has a stationary bed of desiccant material in a housing provided with a valving arrangement automatically controlled by a program timer such that air to be dehumidified or regeneration air from outdoors is moved through the desiccant bed at different times by a single blower. During the time period that regeneration air flows, an internal electrical resistance heater intercepts and heats the regeneration air as it enters the bed. This raises the regeneration air temperature to about 275.degree. F. which then heats the desiccant material which, in turn, finally heats the adsorbed moisture from its liquid phase to a vapor phase. Thus, the desiccant material is utilized as a susceptor for transferring thermal energy by conduction to the adsorbate. The vapor is taken up by the flowing regeneration air and discharged therewith outdoors from the housing as effluent. Then, under control of the program timer, the electrical resistance heater is switched off and the flow of regeneration air from and to outdoors through the bed is continued for a few minutes to cool the hot desiccant material before switch-over of the valving arrangement for movement of the air to be dehumidified through the bed of regenerated desiccant material, hence for the commencement of dehumidification.
Another conventional dehumidifier/regenerator is designed for continuous duty and is a rotary type which is essentially divided into two parts. One part is a dehumidifying compartment in which humid air is passed through an adsorptively potent half or more of a continuously rotating bed of the desiccant material, where it is dried, and it is then discharged to a controlled region requiring low humidity control of its atmosphere. As the desiccant bed becomes saturated, it passes into the other part of the dehumidifier, this being an adjoining regeneration compartment where, in the regeneration manner of the first-mentioned design, outside air is heated by an electrical resistance heater, passed through the desiccant bed to relieve the bed material of its adsorbed moisture, and discharged back to the outside atmosphere as a hot moist air effluent. The bed material is regenerated at the same rate at which it was being saturated in the dehumidifying compartment, so that the quality of the dry air discharged from the dehumidifying compartment to the controlled region is constant. The relative movement between the bed and the compartments, of course, may be carried out with rotation of the compartments instead of the bed.
Each of these conventional designs has been modified heretofore by replacing its electrical resistance heater with steam coils or with a direct-fired or indirect-fired gas or oil burner. The heated regeneration air, in any case, still first heats the saturated desiccant material, whereafter the hot desiccant material transfers some of the heat to the adsorbed or surface moisture in its pores and capillaries so as to vaporize the moisture. After the vapor is purged from the bed, it moreover still becomes necessary to cool the regenerated, yet hot, bed material. Thus, it is evident that a considerable expenditure of energy and time is necessary for carrying out the regeneration of desiccant material used in desiccant dehumidifiers of, or functionally similar to, the foregoing conventional designs. This poses a problem as to how one might carry out such desiccant regeneration much more efficiently, so that a highly significant reduction in the expenditure of energy and time will be achieved in the regeneration process.
The use of microwave energy for heating, vaporizing and removing moisture from grains of plastic resins to be used in molding processes, or from grains of organic material to be used in food or agricultural products, is taught in U.S. Pat. Nos. 3,834,038; 4,023,279; 4,330,946; 4,332,091; and 4,430,806. Of these, only U.S. Pat. No. 4,023,279 mentions a desiccant bed and his is solely in connection with its suggested use for supplying hot dry air said to be essential for picking up moisture vaporized out of wet resin particles by microwave energy as the particles drop from a hopper counter-current to the air. In fact, U.S. Pat. No. 4,023,279 teaches that drying resin particles by microwave energy and hot dry air eliminates the need, within the enclave of the drier, of a desiccant bed having the disadvantage of losing its effectiveness after saturation.